A novel polyorganosiloxane/polyvinyl-based graft polymer has been discovered which is extremely useful as a modifier for various thermoplastic resins while maintaining many unique features of silicone rubber.
It shows good blend morphology, good rubber dispersion and no tendency to agglomerate, all as shown by transmission electron microscopy (TEM), while imparting good impact strength and ductility at a wide range of temperatures with no tendency to cause delamination and surface mottling to articles molded from thermoplastic resin blends containing this modifier.
There have been many attempts in the art to provide polyorganosiloxane-based graft polymers which may be useful as impact strength modifiers for thermoplastic resins. See for example, U.S. Pat. No. 2,891,920 (J. F. Hyde, et al.); and O. Graiver, et al., Rubber Chem. Tech., 56 (5), 918 (1983).
The major deficiencies which have prevented the widespread use of polyorganosiloxane impact modifiers in thermoplastic resins have included raw material costs, relatively poor rubber integrity, and the incompatibility of the silicone-based rubber modifier and the thermoplastic resin. Additionally, the siloxane polymerization process requires careful control to eliminate contamination of the silicone rubber by linear or cyclic siloxane oligomers. Surface delamination in molded thermoplastic parts has been partially attributed to the presence of such oligomer contaminants in the silicone rubber.
Polycarbonate resin compositions and blends of polycarbonate resins with other thermoplastic resins are widely used because of their excellent properties. Polycarbonate resin compositions and mixtures thereof with a saturated polyester and/or a poly(etherester) elastomer with a polyorganosiloxane-based impact modifier are described, for example, in European Patent Application No. 0,249,964. It is disclosed therein that very desirable improvements in chemical resistance, weather resistance and low temperature impact resistance are achieved if an additive comprising a silicone-based graft polymer is used.
Polyphenylene ether resin compositions alone or in blends with other resins are also widely used because of their excellent properties. Polyphenylene ether resin compositions and mixtures thereof with a polystyrene resin along with polyorganosiloxane-based modifiers are described, for example, in European Patent Application No. 0,260,552. It is disclosed therein that very desirable improvements in impact resistance, heat resistance, mechanical strength, surface appearance, and moldability and flow properties are achieved if an additive comprising a silicone-based graft polymer is used.
Both of the above references used a modifier comprising a silicone rubber onto which a vinyl monomer(s) is grafted in the presence of a graft-linking agent. Mention is also made of European Patent Application No. 0,246,537 which also describes the use as an impact modifier of a polyorganosiloxane polymer substrate on which are subsequently grafted first and second vinyl-based polymer stages. European Patent Application No. 0,260,552 also describes soaking the first stage substrate with the second stage monomer(s) to cause an "entangling" thereof with the silicone prior to subsequently polymerizing the second stage. Such modifiers have relatively poor rubber integrity and incompatibility with the resins. This may lead to poor surface appearance and delamination in the molded articles.
Also relevant for its broad teachings is BASF's U.K. Patent No. 1,590,549 which also describes a silicone rubber graft polymer for thermoplastics and particularly Example 6 which discloses a composition comprising 90 weight percent of polycarbonate and 10 weight percent of the graft polymer discussed immediately above; poor compatibility is observed with all of these compositions.
Each of these disadvantages can be overcome by the practice of the present invention where polyorganosiloxane rubber is replaced by a co-homopolymerized substrate(s) of polyorganosiloxane/polyvinyl-based polymer(s) which is subsequently graft polymerized with a (meth)acrylate polymer, a vinyl aromatic/(meth)acrylate copolymer or a vinyl aromatic/vinyl cyanide/(meth)acrylate terpolymer with or without an intermediary stage comprising at least one polymer or at least one cross-linked vinyl polymer or mixture thereof.
These grafted (meth)acrylate polymers exhibit no agglomeration, and blends comprising compounded polycarbonate resins (PC), PC/poly(1,4-butylene terephthalate) (PBT) resins, PC/PBT/polyphenylene ether (PPE) resins, PBT/PPE resins, PC/PPE resins, or mixtures of any of the foregoing mixtures containing the graft polymers of the present invention exhibit enhanced impact strength at a wide variety of temperatures and particularly at low temperature; good tensile strength, surface appearance, and weld lines; good rubber dispersion; a highly desirable blend morphology with the modifier appearing as separate spherical particles; no delamination; and no agglomeration; while maintaining other properties, such as weathering and thermal resistance.